1. Field of the Invention
Technical Field
The invention relates to analog to digital converters using a delta-sigma modulator and particularly to one having very low distortion and noise where power dissipation is a concern.
2. Description of Related Art
Amplifiers are well known in the art. For high precision switched capacitor circuits and ADC""s the class A operational amplifier is well suited. A class A amplifier dissipates a constant amount of power independent on the input or output conditions. This is well suited for low-distortion systems, but poor from a power dissipation point-of-view.
Integrators are also known in the art. Some integrators are passive, in that they are made up of only components such as resistors or capacitors. Other integrators are active, using an amplifier to transfer the signal to an integration element, usually a capacitor. For low distortion, low noise applications, the active integrator is best suited. With CMOS integrated circuits, the switched capacitor integrator, and more generally, the switched capacitor filter is a commonly used circuit. In a switched capacitor circuit, a voltage is sampled on a capacitor in one phase, and the resulting charge is transferred in a second phase. This repeated moving of charge packets results in a current flow. This switched capacitor xe2x80x9cbranchxe2x80x9d behaves much like a resistor when viewed at a low frequency. Its advantage in CMOS integrated circuits include manufacturability and matching to other elements. Delta-sigma modulators are also known which provide a series of binary signals at an output which in a certain frequency range is a digital representation of an input signal.
Systems for conducting seismic exploration are well known in the art. On land, a plurality of transducers are deployed over a region and configured to receive reflections of acoustic signals from different geophysical layers beneath the surface of the earth. Seismic sensors are connected over cables to signal conditioning, digitization and digital recording equipment. When utilizing a seismic system, a strong acoustic signal is generated by, for example, setting off an explosion or by utilizing an acoustic signal generator having a relatively high power output. Reflections of the acoustic signals from the geographical layers are then received at the seismic sensors deployed over a analysis.
One problem with seismic exploration is that it frequently occurs in remote areas. As a result, transportation becomes a problem. Such remote areas typically do not have sources of electrical power. Accordingly, when undertaking seismic exploration in a remote area, electric power must be transported in. Whether the transportation occurs by air or by people physically hiking into a rugged area, weight is a significant factor. A common form of power source utilized in seismic exploration makes use of batteries. Batteries are generally heavy. As a result, any power saving that can be achieved results in significantly reduced costs for a particular exploration.
When seismic exploration is undertaken over water, commonly an array of seismic sensors is towed behind a boat using cables which can extend over a mile in length. Like on land, an acoustic generator is utilized to generate an acoustic impulse, reflections of which occur at geophysical boundaries. Those reflections are detected by the seismic sensors towed behind the boat and recorded, typically, for later analysis. In any seismic environment, it is important to reproduce the captured signals with great precision to insure that the information of interest can be reliably obtained. Like on land, power dissipation is a concern in the marine application. This is because of the problem distributing power over the length of the towed cable.
A high performance, low power consumption analog to digital converter is shown which uses a delta-sigma modulator having filter coefficients optimized for reduced power consumption and which uses an amplifier supplied with different levels of power during different operational phases.